Groundwater nitrate 2007-2023

Units

mgNO₃/l

Methodology

The indicator analysis is based on annual mean concentrations per monitoring site. Unless the country reports aggregated data, the aggregation to annual mean concentrations is done by the EEA. The data undergo automatic and manual quality checks to remove errors and outliers. Monitoring site time series from the same water body are combined and gap filled before further analysis. Gap filling is necessary to ensure that the average European time series represents the same water bodies every year. Water body time series are only included in the analysis if they have data within the first and last five years of the time range and data from at least 40% of the years in the time range.

Data are aggregated from site to water body level and gap-filled using generalized additive models (GAM) in the free software R, using the mgcv package. For water bodies with more than one site, a GAM model with concentration as response variable, year as fixed effect and site as random effect is used to estimate annual concentrations per site. Annual values for the water body are subsequently estimated by averaging the annual site predictions. For water bodies with only one site, or where the random effect for site is non-significant, a GAM without a random site effect is used to estimate annual values and for gap filling. The gap-filled water body time series are averaged by country and a weighted average of the country time series (weighting by the total length of rivers or total area of lakes/groundwater in the country) is calculated to give the time series for Europe. A 95% confidence interval for the weighted average is calculated using of the formula for standard error of a weighted mean recommended in Gatz and Smith (1995)